1. Field of the Invention
The present invention relates generally to a lithographic apparatus. More specifically, the present invention relates to a method for joining at least a first member and a second member of the lithographic apparatus.
2. Description of Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device or structure, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction.
A lithographic projection apparatus and device manufacturing method are known from the European patent application EP 1359469, having application number EP 03252747.5, which is incorporated herein by reference in its entirety. The known apparatus comprises a chuck for use in holding a substrate or a mask onto a supporting table by electrostatic force. The chuck comprises at least a first member and a second member, which are joined together. The first and second member each comprise ultra low expansion glass ceramics and ultra low expansion glass, having coefficients of thermal expansion of less than about 0.1×10−6K−1, particularly less than about 0.02×10−6K−1, for example, ZERODUR™ and ULE™.
In EP 1359469, a method for joining the first member and second member is described, wherein the members are joined together using anodic bonding. An advantage of anodic bonding is that materials with strong degassing, for example, glue, are not needed to make the bond between the first and second member, neither is frit fusion applied, nor thermal fusion. In frit fusion, low melting frits (powders) of a glass material are added to a joint to glue pieces together when heated. In thermal fusion, pieces of material are melted at the joint at high temperature. Both frit fusion and thermal fusion have a process temperature which is above the usage temperature of glass ceramics. Therefore, frit fusion and thermal fusion may lead to an undesired degradation of the bonding surfaces of the joined members, for example, from materials or optical point of view.
Anodic bonding results in a firm bond between the ultra low expansion members. Anodic bonding is particularly advantageous for providing strong bonds between ultra low expansion glass ceramic members comprising, for example, ZERODUR™ and ultra low expansion glass members comprising, for example, ULE™. The known method for joining a plurality of members together is applicable, for example, for manufacturing chucks, substrate tables, mask tables, mirrors, and the like.
A disadvantage of the known method for joining at least a first member and a second member is that it has a fair chance of failure. For example, it is relatively difficult to obtain a desired anodic bond between the first and second member having a substantially clean bonding surface. Contamination may, for example, be trapped between the first and second member when the anodic bond has been formed. Such contamination can comprise particles, dust, atomic clusters, and the like. The contamination may impair certain desired properties of the first and second member, for example, optical properties, properties relating to diffraction, reflection and/or transmission of a radiation beam, and the like. The contamination may also impair material properties of the first and second member concerning, for example, a desired homogeneity, crystal structure, and the like. The trapped contamination may lead to material defects, surface distortions, dislocations and/or cracks of the first and/or second member at the time of bonding or after they have been joined together using anodic bonding. Such defects may occur after the anodic bonding, for example, during further treatment or processing of the joined members, such as during a subsequent removal of material from one or more of the joined members. Moreover, the material defects may lead to a further degradation of desired optical properties of the first and/or second member. One or more contaminating particles may damage the first an/or second member such that the anodically bonded members are unusable and have to be discarded, thereby leading to a waste of materials, time and money.
Moreover, it may be desirable to bond the members in a certain alignment with respect to each other. In case the members are found to be misaligned after the bonding process, the members may also have to be discarded, as a result of having become unusable.